We've been going through similar issues around here--mostly my mom not knowing what she wants to be called. My husband's mom is already "Grandmama," and my grandmother is "Nanny." We have finally settled on "LaoLao" for my mom (Chinese for maternal grandmother).

On the same name issue, my husband calls my grandmother by her first name because his late grandmother was also "Nanny," and he isn't comfortable using this name for someone else.

As for my grandmothers, in addition to "Nanny" (which started out as "Granny," but that's another story for another day), my paternal grandmother was "Mamaw." (Can you tell I'm in the deep south? )

Edited to add: My father-in-law is "Granddaddy" and my dad will be "Papa."